Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a cellular system. In the CDMA system, a mobile station communicates with any one or more of a plurality of base stations dispersed in a geographic region. Each base station continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a base station will be simply referred to as a pilot. The mobile station monitors the pilots and measures the received energy of the pilots.
IS-95 defines a number of states and channels for communication between the mobile station and the base station.
For example, in the Mobile Station Control on the Traffic State, the base station communicates with the mobile station over a Forward Traffic Channel, and the mobile station communicates with the base station over a Reverse Traffic Channel. During a call, the mobile station must constantly monitor and maintain four sets of pilots collectively referred to as the Pilot Set--the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set are pilots associated with the Forward Traffic Channel assigned to the mobile station The Candidate Set are pilots that are not currently in the Active Set but have been received by a particular mobile station with sufficient strength to indicate that the associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set are pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set are all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
The mobile station constantly searches a Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. As the mobile station moves from the region covered by one base station to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message. The base station determines an Active Set according to the Pilot Strength Measurement Message, and notifies the mobile station of the new Active Set via a Handoff Direction Message. When the mobile station commences communication with a new base station in the new Active Set before terminating communications with the old base station, a "soft handoff" has occurred.
In the Mobile Station Idle State, the mobile station monitors a Paging Channel, transmitted from a single base station of the Active Set for control information and pages. Furthermore, the mobile station continues to search the Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than the pilot of the currently monitored base station. When a pilot is found to be sufficiently stronger, an "idle handoff" to the neighboring base station having the stronger pilot will occur, and mobile station will then monitor the Paging Channel from this base station.
In a System Access State, the mobile station continues to receive control information over the Paging Channel, and also transmits short signaling messages, such as, call originations, responses to pages, and registrations, over an Access Channel to the monitored base station. While in the System Access State, the mobile station continues its pilot search and updates its Neighbor Set, but, pursuant section 6.6.3.1.3 of IS-95, and in contrast to the Mobile Station Control on the Traffic State and Mobile Station Idle State, the mobile station is not permitted to handoff to another base station.
Whenever the mobile station sends a message in the System Access State, it must make an "access attempt" to the single base station it is monitoring. An access attempt is the process of repeatedly transmitting the message until receiving an acknowledgment for the message from the base station. Each transmission in the access attempt is called an "access probe," which is composed of an Access Channel Preamble and an Access Channel Message. The contents of the an Access Channel Message depends on the type of signaling message. According to section 6.6.3.1.1.1 of IS-95, the same message is sent in each access probe.
IS-95 specifies a particular protocol whenever the mobile station originates a call or responds to a page in the System Access State and sets up for the call in the Mobile Station Control on the Traffic State. Only certain parts of the protocol are discussed herein.
The mobile station first sends an Origination Message over the Access Channel. Upon successful receipt of the Origination Message, the base station sends a Channel Assignment Message over the Paging Channel. The Channel Assignment Message specifies a Traffic Code Channel of the monitored base station for demodulation by the mobile station. The mobile station tunes to the specified Traffic Code Channel, and receives the Forward Traffic Channel. The base station acquires the mobile station on the Reverse Traffic Channel. The mobile station sends, over the Reverse Traffic Channel, the Pilot Strength Measurement Message indicating the current Candidate Set. The base station transmits, over the Forward Traffic Channel, an Extended Handoff Direction Message to the mobile station specifying the Active Set. When the Extended Handoff Direction Message specifies the single, monitored base station and other base stations, a soft handoff is made to the different base stations, and the call is demodulated from the Traffic Code Channels of the base stations represented by the pilots of the Active Set.
Dropped calls and poor call quality reception has been observed using this protocol. This results from the mobile station communicating with only one base station over the Traffic Channel until the soft handoff is made.
As an alleged remedy this problem, a proposed modification to IS-95 would more quickly move the mobile station into soft handoff by reporting pilot signal strengths and specifying certain members of the Candidate Set over the Access Channel rather than waiting until a Traffic Channel is acquired. That is, the mobile station would measure pilot strengths and add to the initial Access Channel Message the identities of base stations that are suitable for a soft handoff. The base station would include the Active Set in the Channel Assignment Message. Consequently, the mobile station will know its Active Set before acquiring the Traffic Channel, and can immediately acquire the Traffic Channel from multiple base stations rather than one.
This proposed solution has a problem when multiple access probes are required to make a successful access attempt and conditions change. The time between successive access probes is in the order of hundreds of milliseconds. During this time the pilot strengths may change. But, because the same message is sent in each access probe, the same pilot strength measurement that is reported by the mobile station in the initial access probe is reported in subsequent access probes. Because the base station receives outdated pilot strength measurement information in the subsequent access probes, it may not specify the best base stations suitable for handoff in the Active Set. Consequently poor call quality or dropped calls ensue.
A need therefore exists for a system, method, and apparatus for soft handoff that provides current information on pilot signal strengths to the base station.